Field of the Invention
The present invention relates to coordinate input apparatuses that optically detect a coordinate location input on a coordinate input surface using a pointing device, such as a finger, for inputting and selecting information. The present invention particularly relates to removable and portable coordinate input apparatuses.
Description of the Related Art
Thus far, various types of coordinate input apparatuses (such as touch panels, digitizers, and so on) have been proposed or commercialized as this type of coordinate input apparatus. For example, touch panels and the like, which allow a terminal such as PC (personal computer) or the like to be operated with ease simply by touching a screen with a finger without using a special tool, have become widespread.
Various coordinate input systems are used in such touch panels, such as panels that employ resistive films, panels that employ ultrasound waves, and so on. A system in which a retroreflective material is provided on an outer side of a coordinate input surface, light from a light projector is reflected by the retroreflective material, and a light amount distribution thereof is detected by a light receptor (an optical shielding system) is known as a system that uses light (see Japanese Patent Laid-Open No. 2004-272353, for example). This method detects a direction of a light-shielded portion (region) shielded from light by a finger or the like within a coordinate input region, and determines the coordinates of a light-shielded position, or in other words, of a coordinate input position. Meanwhile, Japanese Patent Laid-Open No. 2014-48960 discloses a system that improves usability by enabling a coordinate input apparatus to be installed in a desired location.
Integrating this type of coordinate input apparatus with a display device makes it possible to control display states, display trajectories of specified positions as handwriting in the same manner as writing on paper with a pencil, and so on, simply by touching the display screen of the display device.
Various types of flat-panel displays, such as liquid crystal display devices, front projectors, and so on are known as display devices. In the case of a flat-panel display, such an operational environment can be realized by overlaying the coordinate input apparatus thereon, with a mobile device such as a smartphone being a typical example thereof. As flat-panel displays increase in size, such displays are being combined with large-format touch panels, which are now being introduced in fields such as digital signage, for example.
Several types of errors caused by the configuration can arise in such devices. To describe several the main causes of such errors, for example, a light-receiving device such as a charge coupled device (CCD) line sensor, a complementary metal oxide semiconductor (CMOS) sensor, or the like is used as a light receptor that detects light. Light is received by the light-receiving device in units of pixels, and thus when the received light is quantized in units of pixels, quantization error will arise. In addition, table lookup or conversion such as polynomial approximation is used as a method for converting pixel numbers of pixels detected by the light-receiving device into angle values, but some error will occur in either of those methods. Furthermore, in the case where the device is anchored to a housing, a process such as recording a reference angle used in coordinate calculation is carried out. For example, this process measures numbers of pixels indicating an angle with the horizontal direction or the like used as a reference when the apparatus is assembled, and records that number in a memory in the apparatus. Measurement error or the like occurs at this time. Furthermore, it is also conceivable that error will occur as placement positions in the apparatus shift due to changes over time. These various causes of error will result in the angles detected by the light-receiving device containing error as well.
If the angles detected by the light-receiving device contain error, a problem will arise in that the coordinates calculated on the screen will not match the touched position. In order to reduce the difference between the two, it is necessary to reduce the error that occurs by increasing the pixel resolution of the light-receiving device, improve the mechanical precision of the apparatus, and so on. In reality, design levels are determined in consideration of, among other things, a balance between costs and functions/performance.
Referring to, for example, FIG. 13B of Japanese Patent Laid-Open No. 2014-48960, if error is present when θ17 or θ18 is an acute angle, error in the calculated coordinates takes on a high value, and the position actually input is shifted relative to the touched position as a result. This can result in a drawing position being shifted, a pointer being shifted, or the like, for example, which in turn can make it impossible to carry out desired operations, such as clicking and selecting objects and so on.